222 CHEMICAL ENGINEERING The term classifying is used for the grading effect obtained on subjecting a granular or powdered mass to the action of a current of liquid in a tank or other device for giving the same effect. This produces a gradation from coarse to fine in the enclosing tank or other kind of apparatus used. Any length of the settled particles can be taken as a grading and the contamination of the coarse particles by the very fine ones due to eddying currents or the clinging'of the fine particles to the coarse ones in the descent can be obviated to some extent by constricting the discharge openings placed near the bottom of the settling apparatus used and introducing through such constricted opening an upward rising current of liquid. The term can also be used for settlement and siphoning operations without flow and where the time of settlement is progressively increased. This is the standard method used in the abrasives industry for producing very fine powders. Separation.—This is a term much used by inventors synonymously with the others given. In this book its use is confined to operations and devices for separating solids from solids, solids from liquids, solids from gases and liquids from liquids. The great commercial importance of the centrifugal separator sanctions the use of the heading to the operations performed by this machine or operations where two products issue or arise, one often being waste. In addition to centrifugal separators, filtering apparatus will be considered under this head, also the so-called "classifiers" which have been developed in Western ore treatment practice. These devices separate in water ore which has been finely ground into sands and a finely divided part known in ore treatment as slime. The name classifier applied to them is firmly entrenched. Dry separators for splitting dry materials into fine and coarse parts will also be described under this head. Discussion of separation will be found in Section VII. Concentration.—Under this head is comprised the operations of differentiating a mass of ore, earthy material or other substance according to the specific gravity of the components of such a mass. Usually one or more merchantable products of greater specific gravity than the original average are obtained from the parent mass and the balance of it of low specific gravity is the refuse, waste or tailing. In concentrating operations on some substances such as coal from the mine the merchantable product has less specific gravity than the refuse. A short definition of concentration would be "The art of enriching ores earthy or other substances by machinery.'' The full discussion of concentration is given in Section VIII. Grading.—The processes under this head have mostly to do with fragmental granular or powdered masses of solid material and the means of splitting up a comminuted mass according to size. Means for reducing them to this state from larger unit masses will be found discussed under the head of "Crushing and Grinding." Some reasons for dividing or splitting up a loose or broken muss of material into a number of sizes will be gathered from the following examples of the need of this operation in certain industries. In the use of abrasives the aim is to produce a smooth or polished surface from a roughened one. To gain this the roughened surface must be reduced progressively by abrasive grains whoso diameter is roughly proportional to the depth of the projections of the roughened surface. Quite evidently while the abrasive grains are tearing away the projections of the surface, they are grooving the smoother portions to an amount proportional to the size of grain used, though to a lesser degree. On this account, if the ultimate aim is a smooth or polished surface, the abrasive must be used in graded masses starting with a coarse size and ending with a fine.